In 3GPP (3rd Generation Partnership Project) which is a project aiming to standardize mobile communication systems, the introduction of Device to Device (D2D) communications is discussed as a new function after Release 12 (see Non Patent Document 1).
In the D2D communications, a plurality of user terminals close to one another perform direct radio communications in a frequency band assigned to a mobile communication system. In addition, the D2D communications is also called Proximity Service communications.